1. Field of the Invention
The present invention relates to pumps for pumping fluids such as blood that are sensitive to mechanical working or shear stress. More particularly, the present invention relates to a pump apparatus having an impeller that is magnetically suspended and rotated by electric and permanent magnets with no mechanical contact between the impeller and any other part of the pump.
2. State of the Art
There are many types of fluid pumps suitable for use in a wide range of applications, all performing the same basic function of moving fluid from one point to another, or moving a fluid from one energy level to another. However, pumps for pumping sensitive fluids, such as blood, introduce special design requirements. Additionally, pumps for implantation in a human patient for long or short-term use as ventricular assist devices (VAD's) or complete heart replacement, add additional size, weight, durability, and other requirements.
The design problems associated with sensitive fluids, including blood, generally relate to problems caused by contact of the fluid with mechanical parts and other substances present in the pump. Problem contact areas for sensitive fluids may include 1) contact with materials and structures in rotating fluid seals, 2) contact with mechanical bearing assemblies that are exposed to the fluid, and 3) use in bearing structures that depend on a layer of fluid between moving surfaces to provided reduced friction, such as hydrodynamic bearings. For example, it is well known that rotating shaft seals are notoriously susceptible to wear, failure, and even attack by some fluids. Many types of pumps may also increase mechanical working of the fluid and precipitate detrimental processes such as chemical reactions or blood clotting.
It is also well known that pumps for corrosive fluids, blood, and fluids used in food processing require careful design of the flow passages to avoid fluid damage, contamination, and other undesirable conditions. For example, ball bearing and other rolling element bearings must in general be used with some type of shaft seal to isolate the fluid from the bearing for the above mentioned cases. This may be needed to prevent damage to the bearing by caustic fluids, or to prevent damage to the fluid by the rolling elements of the bearing. For example rolling element bearings can crush and destroy the living cells in blood. Thus, rolling element bearings are generally not practical for blood pumps.
Finally, the size, weight, biocompatibility, and operating durability and reliability of blood pumps are a major concern where VAD's and heart replacement pumps are concerned. It would be desirable to have a VAD or heart replacement pump that can operate reliably for 20 or 30 years despite the normal bumping and jarring of everyday life, including unexpected impact such as from falling, yet is small enough to implant easily in a patient's chest. It is also desirable to reduce the power requirements of such a pump so as to increase mobility of the patient.
To address these problems, pumps with magnetically suspended impellers have been developed. For example, Oshima et. al. (U.S. Pat. No. 5,111,202) discloses a pump in which the impeller is magnetically suspended or levitated within the pump housing, and is magnetically, not mechanically, coupled to the pump housing. The pump employs permanent magnets rotating on a motor external to the pumping chamber, with the external permanent magnets magnetically coupled to opposing permanent magnets on the impeller. Magnetically suspended pumps are well adapted to pumping sensitive fluids because they eliminate the mechanical bearing structure or rotating seals which can damage or be damaged by the fluid.
However, such pumps that are currently known in the art present several drawbacks. First, an external motor with its own means of bearing support (ball bearings) is still required to rotate the impeller. It is the external bearing support that maintains the position of the rotor in such a pump. Though the motor is sealed from contact with blood and other bodily fluids, and is magnetically coupled to the suspended impeller, it still employs bearings which produce heat and pose the potential of failure. Naturally, such pumps tend to be bulky in part because of the size of the electric motor. These pumps are frequently unsuitable for implantation in a human patient because of size, weight, power consumption, and durability problems.
Other methods of magnetically supporting a rotating pump impeller have been developed. Olsen, et. al. (U.S. Pat. No. 4,688,998) teaches a fully suspended pump rotor employing permanent magnet rings on the rotor magnetized along the axis of rotation, and actively controlled electromagnets on the stator that create a magnetic field to stabilize the position of the rotor. This approach also leaves certain problems unsolved. While the manufacture of permanent magnets has advanced substantially, there are still significant process variations. These variations include repeatability from one magnet to the next, and homogeneity of the material within one magnet. The position and stability of the rotor in the Olsen invention is entirely dependent on the homogeneity of the permanent magnet rings. These problems are well known by designers of electro-mechanical devices, where significant steps are normally taken to reduce the dependency of device performance on homogeneous magnets. In the field of permanent magnet motors, this is a well known source of torque ripple.
It would therefore be desirable to have a pumping apparatus with a magnetically suspended impeller that is suitable for pumping blood and other sensitive fluids, and which is small, lightweight, durable, reliable, and has a low power consumption, without using an external motor to drive the impeller. It would also be desirable to have a magnetically suspended pump that has reduced sensitivity to manufacturing process variations in permanent magnets. It would also be desirable to have a magnetically suspended pump that requires no additional sensors for pump status monitoring.